Hitherto, in connection with the screening of drilling muds, vibratory screening apparatus generally was constructed to operate in a single vibratory mode with orbital (circular/elliptical) movement.
The expression "drilling mud" embraces a variety of substances; and the need for screening in this context relates to the separation, from returned mud, of various particles of differing sizes and compositions. This variety has led to the realisation that efficiency of screening drilling muds is related to, inter alia, choosing between orbital and linear vibratory movements. Therefore, there has emerged a demand for vibratory screening apparatus constructed to operate with linear movement.
Generally, it is accepted that out-of-balance vibrator motors provide the best practical and cost-effective means for producing vibratory motion. A single vibrator motor produces orbital movement which is circular or elliptical depending upon the relative positions of the motor and the centre of mass of the apparatus. Two vibrator motors appropriately arranged and rotating in mutually opposite directions produce linear movement. However, whereas two such vibrator motors will self-sychronise to produce linear movement when rotating oppositely, they will not do when rotating uni-directionally.
There is now a need for vibratory screening apparatus operable selectively to perform orbital and linear vibratory movements. One solution for such a selectively-operable or "dual motion" apparatus might be to isolate (switch off) either one of the vibrator motors in a two-motor arrangement, thus converting from linear movement to orbital movement. De-isolating (switching on) the said one vibrator motor would accomplish reversion to linear motion. However, this solution would have the disadvantage that one vibrator motor would be idle during orbital movement and this undesirably would introduce a significant power-rating differential between the respective modes of operation.